Method for writing a pattern on a surface intended for use in exposure equipment and for measuring the physical properties of the surface

ABSTRACT

The present invention relates to a method for writing a pattern on a surface intended for use in exposure equipment, including the steps of: arranging an object having a thickness (T) provided with a surface on a stage of a pattern generating apparatus, dividing the surface into a number of measurement points, where two adjacent measurement points being spaced a distance (P) apart not exceeding a predetermined maximum distance, determining the gradient of the surface at each measurement point, calculating a 2-dimensional local offset (d) in the x-y plane for each measurement point as a function of the gradient, and the thickness (T) of object, and correcting the pattern to be written on said surface by using the 2-dimensional local offset (d). The invention also relates to a method for measuring the physical properties of a surface.

A method for writing a pattern on a surface intended for use in exposureequipment and for measuring the physical properties of the surface.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for writing a pattern on asurface, preferably on a glass plate made from quartz, for use inexposure equipment, as defined in claim 1. The invention also relates toa method for measuring the physical properties of the surface todetermine the shape of the surface of a plate as defined in claim 10.

BACKGROUND TO THE INVENTION

When a large display or part of a display, colour filter or an othersimilar application, is produced, an exposure system transfer an imagefrom a glass plate, preferably made from high quality quarts, onto arather large substrate, which may have a dimension up to 1100 mm times1300 mm or even more. The exposure system includes an aligner, orstepper, that emits light through the glass plate and onto thesubstrate, see FIG. 1. The glass plate is held in place by two rulers,or alternatively by a frame, and therefore the shape of the glass plateis deformed and the aligner, or stepper, compensates for this calculateddeformation. The front side of the glass plate that carries the patternof the image is arranged on the rulers, and a perfect reproduced imageby the system on a substrate is dependent on that the front side of theglass plate is absolutely flat.

It is very important that the registration of masks, i.e. the absoluteplacement in a Cartesian coordinate system, is good enough to permitmasks from different systems to fit together, e.g. the colour filter andthe TFT-array. Furthermore, large TFT substrates may use two or moremasks stitched together to cover a large exposure area.

In pattern generating systems for small plates, a three-foot device isused to support the plate during pattern generation and measurement, butthe weight of a glass plate, with a thickness of 10 mm and a size of1000×1000 mm, is approximately 40 kg, which will not be suitable toplace on three pins. An alternative solution is to use an air cushionfor plate support, but this introduces other problems like determiningthe exact position of the plate during exposure of the pattern. Anotheralternative is to handle the consequences that will arise when placingthe plate directly on the stage (i.e. the support) of a patterngenerating apparatus, although the plate will be deformed.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for writing a patternon a glass plate that is independent of any physical deformations thatwill occur when writing the pattern.

This object is achieved by the method as defined in claim 1.

A further object with the invention is to provide a method for measuringa glass plate being independent of any physical deformations that willoccur when measuring the plate.

This object is achieved by the method as defined in claim 10.

An advantage with the present invention is that unevenness in thesupport of the pattern generating apparatus (or measuring apparatus)will not introduce any error in the pattern or the measurement.

A further advantage is that any unevenness of the back surface and/orthe front surface of the glass plate will not introduce any errors inthe pattern or the measurement.

Still a further advantage with the present invention is thatcontamination in form of particles and/or air trapped between the plateand the support can be compensated for, and therefore will not introduceany error in the pattern or measurement.

Still another advantage is that it is possible to even correct thedeformation that will occur in the exposure equipment together with thedeformation generated during the pattern writing process, provided thatinformation regarding deformation in the exposure equipment is knownwhen manufacturing the plate, as is disclosed in the publishedinternational patent application WO 00/72090 by the same applicant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exposure system according to prior art.

FIG. 2 shows a pattern generating apparatus according to prior art.

FIG. 3 illustrates the plate bending effect for calculating an offsetaccording to the present invention.

FIGS. 4 a and 4 b illustrate the plate bending effect a glass plate witha flat top and a shaped bottom and the introduction of a referencesurface when arranged on a flat support.

FIGS. 5 a and 5 b illustrate the plate bending effect a glass plate witha shaped top and a flat bottom and the introduction of a referencesurface when arranged on a flat support.

FIGS. 6 a and 6 b illustrate the plate bending effect a glass plate witha flat top and a flat bottom and the introduction of a reference surfacewhen arranged on a shaped support.

FIGS. 7 a and 7 b show measured x-y coordinates of a glass plate andcompensated x-y coordinates of the same glass plate using the correctionfunction, and FIG. 7 c shows the difference between the measurementswithout compensation and the measurements with compensation.

FIG. 8 shows a three-dimensional measurement of a glass plate withparticles distorting the shape of the plate.

FIGS. 9 a and 9 b show measured x-y coordinates of the glass plateillustrated in FIG. 8, and the compensated x-y coordinates of the sameglass plate using the correction function.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an exposure system 10 which uses a glass plate 11 restingon two rulers 12. The weight of the glass plate will cause the glassplate 11 to bend when placed on the rulers 12. The deformation of theglass plate caused by the weight is easy to calculate and can becorrected for. The glass plate 11 is provided with a pattern arranged onthe downwards pointing surface 13 resting on the rulers 12. A lightsource 14 emits light 15 onto the glass plate 11 and the patternarranged on the surface 13 of the glass plate 11 will produce a copy ofthe pattern on a substrate 16. The substrate 16 could be a TFT intendedfor a TV monitor. Normally, the pattern is transferred to the substrate16 in a one-to-one relationship.

Other necessary optics is not shown in FIG. 1, since the purpose of thefigure is to describe the function principals, rather than a completeexposure system.

FIG. 2 shows a pattern generating apparatus 20, which also could be usedas a measuring apparatus, including means to write a pattern 21, e.g.mirrors directing a laser beam from a laser, and means 22 to measure theheight H_(z) between the apparatus 20 and a glass plate 11 with thesurface 13 on which the pattern is to be written is placed upwards on asupport 23, so called stage. The pattern writing means 21 may betranslated over the entire surface of the stage, which movement may beimplemented in a number of ways. FIG. 2 illustrates one way where thestage is provided with means to move it in relation to the patternwriting means 21 in the x direction, and where the pattern writing means21 is attached to a sliding support 24 arranged on a beam 25 to move thepattern writing means in the y direction. Other possible ways toimplement the translation of the pattern writing means is to provide themeans to move the stage in both x and y direction with a non-movingpattern writing means, or the pattern writing means could be providedwith means to move in both x and y direction with a non-moving stage.

The apparatus 20 is also provided with an angled foot plate 26 arrangeda constant distance above the surface 13 of the glass plate 11 by meansof an air cushion 27. The foot plate 26 and the pattern writing means 21are attached to the sliding support 24 via a flexible attachment 28, toallow the distance between the sliding support 24 and the patternwriting means/foot plate to vary dependent on the roughness of thesurface 13 of the glass plate 11. The varying distance in the zdirection, i.e. the height H_(z), may be measured to calculate theroughness of the surface 13 in the z direction. The size of the footplate that is parallel to the surface 13 of the glass plate 11 has anopening for a laser beam from the pattern writing means 21 and ispreferably rather large, e.g. 5 mm on each side, since the purpose ofthe measurement is to detect deviations in height over a relativelylarge distance. The air cushion beneath foot plate will act as an autofocus device for the pattern generating apparatus due to the constantdistance between the foot plate and the glass plate. The inventionshould however not be limited to this kind of pattern generatingapparatus using an air cushion as an auto focus device, but other typesof systems that will provide focus for the system could be used. Theessential part is that the apparatus 20 is provided with means tomeasure the height H_(z) between the apparatus and the surface 13 of theglass plate 11 and thereby the variation in height when the patternwriting means 21 is moved in relationship to the stage 23, and thus thesurface 13.

An essential part of the invention is to determine a reference surfaceagainst which the difference in height H_(z) is calculated. Thisdifference is denoted H, as is illustrated in connection with FIG. 3.The reference surface could have any desired shape as long as the shapeof the reference surface is maintained unchanged. Preferably, the shapeof the reference surface is a flat plane.

If it were possible, it may have been desirable to use the “free” (nongravity) form, i.e. the centre line of the plate as a reference surface,which is rather difficult to achieve in practise. The bottom surface ofthe plate is not a good alternative for a reference surface since astepper or an aligner use the top surface as a reference.

On the other hand if the top surface would be used as a referencesurface, there is an additional need to know the bottom shape of theplate and the shape of the support. The shape of the support may beobtained, but it is very difficult to achieve knowledge of the bottomsurface in practice. The top surface may however be measured without theknowledge of the bottom surface. A large glass plate that is placed on athree-foot will be deformed due to the weight of the plate, but adeformation function for a perfect plate may be calculated if thethickness of the plate, the material of the plate and the configurationof the three-foot are known. A measurement of the non-perfect glassplate, when placed on the three-foot, will generate a measurement of thedeformed plate. The shape of top surface is then calculated bysubtraction the calculated deformation function for a perfect plate fromthe measurement of the deformed plate.

The top surface of a glass plate is normally much more even, i.e. lessvariation in height in relation to the centre line, compared to thebottom surface, and the best compromise should therefore be to make thetop surface of the plate to be the reference surface. It should howeverbe noted that it is not evident that the top surface is the best choicedue to the deformation of the glass plate during the following step inthe exposure system, as shown in FIG. 1. If the top surface 13 of theglass plate exhibits variations close to the position where it rests onthe rulers 12, the pattern on the surface 13 will be distorted in avicinity of the rulers 12.

It should however be noted that any surface may be used as referencesurface, although the top side is preferred.

FIG. 3 illustrates the plate bending effect for a glass plate 11 havinga thickness T. A reference surface 30 is determined, in this example thereference surface is flat, and the glass plate is divided into severalmeasurement points 31 and the height H_(z) is measured at eachmeasurement point by the means 22 shown in FIG. 2. The height H betweenthe reference plane 30 and the deformed surface 13 of the glass plane 11can easily be calculated by subtracting the height of the referencesurface 30 at the measurement point from the height H_(z) measured forthe surface 13 of the glass plate 11 by the apparatus 20.

A local offset d (as a function of x and y) is thereafter calculated foreach measurement point and depends on three variables: the thickness ofthe glass plate (T), the distance between adjacent measurement points(P) and the measured height (H) between the reference surface 30 and thesurface 13 of the glass plate 11. The local offset should be interpretedas the position deviation from the position where a pattern should bewritten in relationship to the reference surface, as described inconnection with FIGS. 4-6. The pitch P on the surface of the platediffers from the nominal pitch P_(nom) on the reference surface.

The distance between adjacent measurement points should not exceed apredetermined distance, which is dependent on the required accuracy forthe measurement to get a reasonable good result from the measurement. Anexample of maximum distance between adjacent measurement points is 50 mmif the thickness of the glass plate 11 is around 10 mm and the glassplate material is quartz. The distance between adjacent measurementpoints also vary dependent on the thickness of the glass plate to obtainthe same measurement accuracy. The variations in thickness of the glassplate is may be around 10-15 μm, but could be larger. The measurementpoints could be randomly distributed across the surface 13, but arepreferably arranged in a grid structure with a predetermined distancebetween each point, i.e. pitch, that is not necessarily the same in thex and y direction.

The local offset is a function of the gradient in x and y direction ateach measurement point and could be calculated using very simpleexpressions.

An angle α may be calculated from the measured height H provided thedistance P between two adjacent measurement points 31 a is known.

For small angles α: $\alpha = \frac{H}{P}$

Furthermore the local offset d may be calculated provided a is smallusing the formula: $d = {{\frac{T}{2}*\alpha} = \frac{H*T}{2*P}}$

It should however be noted that the formula for calculating the localoffset d above, only is a non-limiting example of a calculation todetermine the offset d. The gradient in each measurement point could bedirectly measured by the system and the local offset is proportional tothe gradient and the thickness of the plate.

As previously mentioned above, FIG. 3 illustrates the bending effect inone dimension, but the local offset d is a 2-dimensional function of thederivative in each measurement point (dx and dy).

As a non-limiting example we assume that the distance between twoadjacent points 31 is 40 mm, the thickness of the glass plate is 10 mm,and that the measured height H is 1 μm, which will result in aone-dimensional local offset d of 125 nm.

FIGS. 4 a and 4 b illustrate the plate bending effect a glass plate 41with a flat top surface 43 and a shaped bottom surface 42 and theintroduction of a reference surface 44, which is flat in this example,when supported by a flat support 45.

When the glass plate 41 is arranged on the flat support 45, the shape ofthe top surface 43 is changed and the bottom surface 42 will generallyfollow the flat support 45. The result of this is that the patterngenerated, illustrated by the dots 46 on the top surface, has to beexpanded to obtain a correct reference surface.

FIGS. 5 a and 5 b illustrate the plate bending effect a glass plate 51with a shaped top surface 53 and a flat bottom surface 52 and theintroduction of a reference surface 44, which is flat in this example,when arranged on a flat support 45.

When the glass plate 51 is arranged on the flat support 45, the shape ofthe top surface 43 is unchanged and the bottom surface 42 will followthe flat support 45. The pattern generated, illustrated by the dots 55on the top surface, has to be expanded to obtain a correct referencesurface, since the top surface will be flattened out when positioned inthe exposure equipment as described in FIG. 1, at least in the vicinityof the rulers 12. The part of the glass plate positioned right betweenthe rulers 12 will be deformed. Furthermore the rulers will deform thepattern on the glass plate unless the shape of the rulers 12 is inaccordance with the shape of the reference surface.

FIGS. 6 a and 6 b illustrate the plate bending effect a glass plate 61with a flat top surface 43 and a flat bottom surface 52 and theintroduction of a reference surface 44, which is flat in this example,when arranged on a shaped support 62.

When the glass plate 61 is arranged on the shaped support 62, the shapeof the top surface 43 is changed and the bottom surface 42 willgenerally follow the shaped support 62. The pattern generated,illustrated by the dots 64 on the top surface, has to be expanded toobtain a correct reference surface, since the top surface will beflattened out when positioned in the exposure equipment as described inFIG. 1.

FIGS. 4 a-4 b, 5 a-5 b and 6 a-6 b illustrate extreme conditions and inreality all three variations are present during the process of writing apattern on a glass plate.

The overall error is however much smaller since all errors from thebottom surface, support surface and contamination, see FIGS. 8, 9 a and9 b, are eliminated or at least reduced.

FIG. 7 a shows measured x-y coordinates of a reference glass plate andcompensated x-y coordinates of the same reference glass plate using acalculated correction function according to the present invention. FIG.7 b shows the measured height H (z correction data) obtained at the sametime as the x and y coordinates for marks depicted on the surface of thereference glass plate. FIG. 7 c shows the difference between themeasurements without compensation and the measurements withcompensation.

The size of the glass plate is in this example 800×800 mm, and thedistance between each dashed line 70 in FIG. 7 a is 50 mm, and the scaleof the deviation of the two plotted charts are 500 nm between eachdashed line 70. The grey lines 71 correspond to the measured deviationof the x and y coordinate on the reference glass plate. The black lines72 correspond to the compensated x and y coordinates of the samereference glass plate using the Z correction effect based on themeasured height H shown in FIG. 7 b. The minimum height is −20.705 μmand the maximum height is +16.664 μm compared to the determinedreference surface and the height H is depicted as a function 73. Thedistance between the lines in x and y direction is the same as in FIG. 7a, i.e. 50 mm, and the distance between the lines in z direction is 2μm.

FIG. 7 c clearly illustrates the deviations between the two functions inFIG. 7 a. When comparing the measured height H in FIG. 7 b with thedeviation in FIG. 7 c it is easy to see the relationship between thederivative of the height and the local offset. When the derivative ofthe height is zero, as in position 74, then the local offset d is zero.When the derivative of the height is high, as in position 75, then thelocal offset d is large.

A transition from a low H value to high H value corresponds to that theglass plate has a “negative” bend, as illustrated in FIG. 3, and viceversa. The calculated local offset, i.e. the difference between the greyand the black lines is largest when the change of the derivative of theheight H in x and y direction is the highest.

FIG. 8 shows a three-dimensional measurement 80 of a glass plate withtwo present particles, placed between the plate and the support, havinga height of 16 μm and 6 μm, respectively. The measurement was performedusing a grid structure and the distance between the measurement pointswas set to 50 mm and the thickness of the plate was 10 mm. The scale inz direction was set to 2 μm per division. The presence of the largeparticle causes the x and y measurement illustrated in FIG. 9 a todeviate more than 500 nm.

FIG. 9 a shows measured x-y coordinates of the glass plate illustratedin FIG. 8, and FIG. 9 b shows the compensated x-y coordinates of thesame glass plate using the correction function calculated from themeasured deviating height measurement in FIG. 8. The effect of particleswill be greatly reduced on the final image generated on the glass plateas is illustrated in FIG. 9 b.

Although a glass plate has been used as an illustrative example in thepatent application, the scope of the claims should not be limited to aplate made of glass.

Furthermore, the pattern generating apparatus could of course includecorrection functions for any repeatable error, e.g. errors present insubstrates for the manufacturing of TFT-arrays that are introduced inthe substrates during the manufacture of the substrates, as well asrepeatable errors introduced in the manufacturing process in thealigner, or stepper as previously mentioned.

The method may naturally be implemented into a computer program forperforming the measurements, and calculating the local offset for eachmeasurement point.

1. A method for writing a pattern on a surface intended for use inexposure equipment, comprising the steps of: arranging an object havinga thickness (T) provided with a surface on a stage of a patterngenerating apparatus, dividing the surface into a number of measurementpoints, where two adjacent measurement points being spaced a distance(P) apart not exceeding a predetermined maximum distance, determiningthe gradient of the surface at each measurement point, calculating a2-dimensional local offset (d) in the x-y plane for each measurementpoint as a function of the gradient, and the thickness (T) of object,and correcting the pattern to be written on said surface by using the2-dimensional local offset (d).
 2. The method according to claim 1,wherein the step of correcting the pattern comprises the steps:determining a correction function for the surface using the calculated2-dimensional local offset (d) for each measurement point, and writingthe pattern on the surface using the correction function with thepattern generating apparatus.
 3. The method according to claim 1,wherein the step of determining the gradient comprises measuring thevariation in height of the surface at each measurement point.
 4. Themethod according to claim 3, wherein the step of measuring thevariations in height of the surface comprises the steps of: determininga reference surface, measuring the height (H) between the referencesurface and the surface of the object at each measurement point, wherebythe 2-dimensional local offset (d) in the x-y plane may be calculated asa function of the measured height (H), the distance (P) from each atleast one adjacent measurement point, and the thickness (T) of theobject.
 5. The method according to claim 4, wherein the local offset (d)is calculated using the formula:d=(T*H)/(2*P)
 6. The method according to claim 3, wherein themeasurement points are arranged in a grid structure having a firstpredetermined pitch in the x direction and a second predetermined pitchin the y direction.
 7. The method according to claim 4, wherein theheight (H) between the reference surface and the surface of the objectoriginate from unevenness of the stage, and/or unevenness of one or bothsurfaces of the object and/or undesired objects arranged between thestage and the object.
 8. The method according to claim 7, wherein theundesired objects may be trapped air or particles.
 9. The methodaccording to claim 1, wherein the top surface of the object is selectedto carry the pattern.
 10. The method according to claim 1, wherein thecorrection function also compensates for expected deformation from theexposure equipment during subsequent processing steps.
 11. A method formeasuring the physical properties of a surface, including the steps of:arranging an object having a thickness (T) provided with a surface on astage of a measuring apparatus, dividing the glass plate into a numberof measurement point, where two adjacent measurement points being spaceda distance apart not exceeding a predetermined maximum distance,determining the gradient of the surface at each measurement point,calculating a 2-dimensional local offset (d) in the x-y plane for eachmeasurement point as a function of the gradient, and the thickness (T)of object, and determining a correction function for the surface usingthe calculated 2-dimensional local offset (d) for each measurementpoint.
 12. The method according to claim 11, wherein the step ofdetermining the gradient comprises measuring the variation in height ofthe surface at each measurement point.
 13. The method according to claim12, wherein the step of measuring the variations in height of thesurface comprises the steps of: determining a reference surface,measuring the height (H) between the reference surface and the surfaceof the object at each measurement point, whereby the 2-dimensional localoffset (d) in the x-y plane may be calculated as a function of themeasured height (H), the distance (P) from each at least one adjacentmeasurement point, and the thickness (T) of the object.
 14. The methodaccording to claim 11, wherein the object is a reference object, andsaid surface is provided with marks at each measurement point.
 15. Acomputer program for performing the following steps: determining thegradient of the surface at each measurement point being defined on asurface of an object having a thickness (T), calculating a2-dimensional, local offset; (d) in the x-y plane for each measurementpoint as a function of the gradient, and the thickness (T) of object,and determining a correction function for the surface, or correcting apattern to be written on said surface, using the calculated2-dimensional local offset (d) for each measurement point.
 16. Acomputer program product for carrying the computer program as defined inclaim 15.